The goal of this proposal is to expand on our current studies showing that significant changes in reactive oxygen species (ROS) metabolism occur in leaf tissue as Arabidopsis plants undergo the fundamental developmental transition from a vegetative state to a reproductive state. This transition, referred to in this proposal as the bolting transition, involves a conversion of the principal shoot apical meristem from one giving rise to leave to one forming a floral meristem which produces a long bolt with terminal flowers. We have demonstrated that the bolting transition is associated with a decline in measured ascorbate peroxidase (APX) activity and with a concomitant increase in lipid peroxidation in leaf tissue. The APX decline and subsequent peroxidation are hypothesized to provide intracellular signals initiating the ordered cascade of gene expressing leading to senescence. In this proposal, experiments are described that will test particular mechanisms of the bolting transition-specific alterations in leaf APX and lipoxygenase activities. In addition, experiments are proposed to exploit the molecular genetic advantages of the Arabidopsis system to investigate the genetic cascades from bolting to senescence by isolating bolting transition-induced genes and screening for late senescing mutant phenotypes. The following specific aims are proposed: 1) Analyze the bolting transition specific down-regulation of leaf ascorbate peroxidase at the transcriptional and post-transcriptional levels; 2) Determine the means of induction of lipoxygenase activity following APX decline; 3) Isolate, sequence, and characterize cDNA clones or genes isolated by differential display whose level of expression changes in leaf tissue immediately following the bolting transition; and 4) Screen mutagenized populations of the ethylene-insensitive Arabidopsis mutant etr1-1 for mutants exhibiting delayed following in response to darkness-induced senescence. Our long term objective is to determine the overall scheme of senescence induction in gene expression program induced as early as the floral transition. Our observation that floral initiation alters ROS metabolism in leaf tissue provides an opportunity to investigate communication between two distinct organs in the plant. We expect that the investigation of these early events will allow the elucidating of the initial signals and the signaling signals and the signaling mechanism response for the life-ending gene expression program known as senescence.